The present invention relates to xe2x80x9cactive implantable medical devicesxe2x80x9d as defined by the Jun. 20, 1990 directive 90/385/CEE of the Council of the European Communities, more particularly to the pacemaker, defibrillator and/or cardiovertor devices, including xe2x80x9cmultisitexe2x80x9d devices, that are able to deliver to the heart low energy stimulation pulses for the treatment of the disorders of the cardiac rate, and even more particularly to the treatment of the disorders of the atrial rate with active implantable medical devices.
In connection with the treatment of atrial rate disorders, certain cardiac stimulation algorithms require discriminating between sinusal atrial depolarization and atrial extrasystoles (AES) of an ectopic origin, before engaging in a corrective action.
AES are in general characterized by a relatively short coupling interval (i.e., an interval of time separating two successive atrial events). To detect those AES with a short coupling interval, the known devices use intervals referred to as windows or refractory periods that are started on the detection of an event (an atrial or ventricular event, according to the type of pacemaker). Any atrial activity that is then detected inside (i.e., during) this interval will be regarded as a AES by the software of the pacemaker. The device then will take a suitable action or not, according to the control algorithms provided.
One can thus envisage two such intervals: (1) a fixed window started on a ventricular event, called xe2x80x9cPVARPxe2x80x9d (post-ventricular atrial refractory period); and (2) a dynamic window started on an atrial event, hereinafter called xe2x80x9cDARRPxe2x80x9d (dynamic atrial relative refractory period). The atrial event starting the DARRP window also starts the application of an atrio-ventricular delay (AVD) interval.
It happens, however, that AES also occur with a low prematurity, i.e., with a relatively long coupling interval. Such an AES, which will be called hereafter xe2x80x9clate AES,xe2x80x9d can occur after the end of the PVARP or the DARRP window and thus be seen by the pacemakerxe2x80x94incorrectlyxe2x80x94as a depolarization of sinusal origin. Taking into account its prematurity, this late AES will be interpretedxe2x80x94still incorrectlyxe2x80x94by the pacemaker control algorithm as an acceleration of the sinusal rate of the patient. This will lead the stimulation control algorithm of the pacemaker to take inappropriate actions.
One can take as an example the particular control algorithm known as xe2x80x9cphysiological overdrivingxe2x80x9d as described in particular in the EP-A-0 880 979 and its corresponding U.S. application Ser. No. 98US-09/079,333, now U.S. Pat. No. 6,078,836 copending and commonly assigned herewith to ELA Mxc3xa9dical, Montrouge, France) the purpose of which is to permanently stimulate the atria after detection of one or more atrial activations of sinusal origin. The pacemaker then increases its stimulation frequency so as to overdrive the otherwise spontaneous rate. But in the event of a late AES, there will result an inopportune increase in the stimulation frequency, which can be badly tolerated by the patient, and in any case is not in conformity with the physiology of the patient.
FIG. 1 shows a ECG strip corresponding to such a situation, where a physiological overdriving algorithm is disturbed by a late AES. In the beginning, the atrial escape interval (xe2x80x9cAEIxe2x80x9d) (i.e., the interval of time, started after a detection or a stimulation in the atrium, at the end of which a stimulation is delivered to the atrium if no spontaneous event is detected) is 780 ms. The overdriving algorithm has calculated on this basis a dynamic window DARRP duration of 585 ms (75% of 780 ms, corresponding to a prematurity rate of AES of 25%). If, as illustrated, a late AES occurs at 600 ms, i.e., with a prematurity of 23%, this late AES will not be recognized as an AES and will instead be seen by the pacemaker as a sinusal atrial depolarization. The reduction in the coupling interval, from 780 to 600 ms, will then be incorrectly interpreted as an acceleration of the sinusal rate, which will cause an abrupt increase in the frequency of atrial stimulation, with the AEI changing from 780 to 550 ms.
The increase of the duration of the PVARP or DARRP windows would not resolve this difficulty, because an excessive lengthening of these windows would be likely to generate a significant number of false positives, i.e., in the event of a spontaneous sinusal acceleration of the atrial rate of the patient, sinusal depolarization would be incorrectly interpreted as an AES, preventing the pacemaker from reacting quickly to the acceleration of the heartbeat rate.
It is, therefore, an object of the present invention to mitigate the aforementioned difficulty, by differentiating the late AES of ectopic origin from spontaneous sinusal accelerations of the patient, and thereby to avoid deluding the control algorithm(s) of the pacemaker by an incorrect interpretation of nature of the atrial depolarization.
For this purpose, the invention thus proposes an improvement to a device of the known type, for example, according to the EP-A-0 880 979 publication mentioned above, including: means for detecting atrial cardiac events; means for applying to the detected atrial events a first window forming an atrial refractory period for the elimination (blanking) of atrial extrasystoles with early or average prematurity, and optionally for the management of the atrio-ventricular delay; and means for reacting to a variation of the sinusal atrial rate detected and filtered by the aforementioned first window.
According to the invention, the foregoing device is improved by also including means for applying to the detected atrial events a second window, distinct from the first window and of a longer duration, forming a relative atrial refractory period for the elimination of the atrial extrasystoles with low prematurity, the duration of this second window being of a variable duration, defined as a fraction of the current average atrial interval; and means for inhibiting temporarily the aforementioned means reacting to a variation of the sinusal atrial rate, in the event of detection of an atrial event inside the second window.
The second window duration, referred to as DLE, can in particular be given by the expression DLE=a *AAI+k; with 0 less than a less than 1, for example, a=0.625; a and k being selected so that the current duration of the second window is less than that of the first window. Advantageously, k is positive and selected so that atrial detections occur systematically inside the second window beyond a predetermined level of heart rate, for example, k=175 ms.
The invention applies preferentially in the case where the first window is a window of the dynamic atrial relative refractory period (DARRP) type.